With the increasing use of mobile communications systems, operators of mobile communications systems need more transmitters, receivers and/or transceivers relaying into cells of the mobile communications systems, such as for example base stations, in order to meet the increased demand for telecommunications.
It is expensive for the operators to increase the number of the base stations. Maintenance of the base stations may, for example, require an engineer to be present at the site of the base station. The increased number of the base stations therefore requires an increased number of service engineers to maintain the increased number of base stations.
With the increased number of the base stations a level of interference is increased between the cells of the mobile communication network. A signal transmitted from a handset of a user outside an individual cell may be sensed as an interference signal by the antenna array of the individual cell. The interference signal is in particular present if users within the individual cell and users outside the individual cell share the same frequency, as is normally the case with modern communication protocols in mobile communication networks.
The antenna array uses a beam to transmit radio signals in a downlink to the handset of the users within the cell. Likewise, the antenna array uses an uplink beam for receiving radio signals from the handsets of the users within the cell. In the prior art the uplink beam pattern and the downlink beam pattern are identical. The prior art teaches as one option an increase in a downlink tilt angle for the antenna array of the cell in order to reduce the interference caused by the base station to users outside the cell. In the prior art the increase in the downlink tilt angle would also cause an increase in an uplink tilt angle. The increase in the uplink tilt angle causes a reduction in interference from the users outside of the cell in the uplink relaying to the base station for the users within the cell. A reduction in coverage area is a trade off when increasing the downlink tilt angle of the downlink beam shape.
With a roll-out of the telecommunications network a network provider is focusing on different aspects during the progression of the telecommunications network. Coverage of the cell, more precisely a sector of the cell is paramount when first setting up the antenna array of the cell. Later in time, the number of the antenna arrays has concomitantly increased with the number of the cells. Therefore the interference caused by users outside the cell is increased. In the prior art it was common to progressively down tilt the antenna arrays in order to reduce the interference from the users outside the cell. It is quite common that radio signals are scattered along a multitude of paths when relayed into the cell. If for example a scattering of radio signals within the cell was substantially changed, it was necessary for a network provider to replace the antenna arrays originally provided by new antenna arrays that are more suitable for the changed scattering conditions present.
WO 99/17576 to Ericsson Radio Systems AB discloses a method and an apparatus for optimizing an antenna tilt angle. The Ericsson patent provides a method for optimizing an advantage of the increased down tilt angle and the concomitant effect of the reduction in the coverage area. The optimum antenna down tilt angle can be identified as the antenna down tilt angle reflecting a maximum interference reduction with respect to a reduction in coverage area. For the Ericsson patent the downlink tilt angle and the uplink tilt angle are identical.
U.S. Pat. No. 6,682,434 to Ericsson incorporates an uplink and downlink transmission quality improvement by differentiated base station antenna beam down tilt angles. A method and an apparatus are disclosed for providing a substantial gain in downlink coverage over interference by means of differentiated beam down tilt angles while still maintaining the coverage limiting signal strength in the uplink by the use of different antenna beam down tilt angles for the transmit and receive antenna beams.